drivers: spi: Add support for half-duplex (3-wire) SPI

Add support for half-duplex (3-wire) SPI operation using the Raspberry
Pi Pico PIO.  To allow control of the size of the driver, including
half-duplex support is optional, under the control of Kconfig options.

The original PIO source code is also included as a reference.

Corrected 3-wire tx/rx counts.

Enable half-duplex code based on DTS configuration

Replace runtime checks with static BUILD_ASSERT()

Signed-off-by: Steve Boylan <stephen.boylan@beechwoods.com>

drivers/spi/Kconfig.rpi_pico

@@ -9,3 +9,26 @@ config SPI_RPI_PICO_PIO
 	select PICOSDK_USE_CLAIM
 	help
 	  Enable driving SPI via PIO on the PICO
+
+if SPI_RPI_PICO_PIO
+
+config SPI_RPI_PICO_PIO_8_BIT
+	bool "PIO 8-bit support"
+	default y
+	help
+	  Enable any addtional code needed to
+	  support 8-bit transfers.
+
+config SPI_RPI_PICO_PIO_16_BIT
+	bool "PIO 16-bit support"
+	help
+	  Enable any addtional code needed to
+	  support 16-bit transfers.
+
+config SPI_RPI_PICO_PIO_32_BIT
+	bool "PIO 32-bit support"
+	help
+	  Enable any addtional code needed to
+	  support 32-bit transfers.
+
+endif